In recent years, a copying machine has been combined with an automatic paper feeding device and a sheet postprocessing device so as to realize automatization of copying and postprocessing operations such as stapling and punching on copied sheets. After images have been copied on the sheets and the sheets have been ejected out of the copying machine, the sheet postprocessing device performs postprocessing operations such as stapling and punching for each set of sheets which consists of a predetermined number of sheets.
Here, with regard to the sheet postprocessing device in which, after sheets have been ejected from the copying machine and have been subjected to a stapling operation that is one of post processing operations, the sheets are ejected onto an ejection tray by an ejection roller since, during an ejection, the edge of the set of sheets tends to rub the surface of the set of sheets that has been ejected earlier and stacked onto the ejection tray, the quality of the fixed image on the surface of the stacked sheets may deteriorate.
Further, during the ejection, if the edge of the set of sheets is caught by a staple of the set of sheets which has been ejected and stacked earlier, the sheets are stacked with disarray.
In order to solve the aforementioned problem, for example, Japanese Laid-Open Patent Publication No.192065/1991 (Tokukaihei 3-192065) discloses a conventional sheet postprocessing device.
In the aforementioned sheet postprocessing device, as shown in FIG. 9, a set of stapled sheets 101 is ejected from sheet ejecting rollers 102 and stacked onto an ejection tray 103 which is installed on the inside of the sheet postprocessing device.
The sheet postprocessing device is provided with sheet detecting levers 104 that have two kinds of lengths for detecting the position of leading portions of both large and small sheets which have been stacked on the ejection tray 103, that is, for detecting the highest position of the stacked sheets on the ejection tray 103. A tray ascending/descending device 106 moves the ejection tray 103 downward so that the highest position of sheets is not more than a fixed line 105. With this operation, during the ejection, the sets of sheets ejected out of the sheet ejecting rollers 102 are not supposed to contact with sheets which have been stacked earlier.
In the conventional sheet postprocessing device, since the ejection tray 103 is installed on the inside of the sheet postprocessing device, the sheet detecting levers 104 are not exposed outside; however, in the case when the ejection tray 103 is installed on the outside of the sheet post postprocessing device, the sheet detecting levers 104 are exposed outside. Thus, the problem is that the long sheet detecting levers 104 spoil the appearance.
The present invention is devised in order to solve the aforementioned problem. The objective is to provide a sheet postprocessing device which positively prevents an uneven image from occurring and keeps the sheets aligned during a stacking operation, without using any means which spoil the appearance, when the sets of sheets are ejected from the sheet postprocessing device onto the ejection tray.